This also fixes a slight inconsistency with raw_post_data after parsing of a
multipart request, and adds a test for that. (Previously accessing
raw_post_data would have returned the empty string rather than raising an
Exception).

This also fixes a slight inconsistency with raw_post_data after parsing of a
multipart request, and adds a test for that. (Previously accessing
raw_post_data would have returned the empty string rather than raising an
Exception).